1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to method and system for performing a handoff between different communication systems.
2. Background of the Related Art
A CDMA mobile communication system typically includes mobile stations that are capable of moving around unspecified regions and base stations for receiving/transmitting messages from/to the mobile stations, as well as managing radio resources. A control station is provided for controlling the base stations and the reception/transmission of messages from/to the base stations, and a switching center provides connections to the control station, switching centers of other communication systems, or other communication networks, for communication between mobile stations or between the mobile station and a fixed station. An important feature of the mobile communication system is the assurance of mobility of the subscriber's mobile station to provide such mobility, the handoff technology is required.
A handoff is a method of switching a communication path between the mobile station and a base station so that communication is continuous when the mobile station goes out of the coverage area of a first base station and into the coverage area of a second base station. A handoff is also used when the mobile station moves beyond a service region of a sector.
There are typically three (3) types of handoffs: a soft handoff, a softer handoff, and a hard handoff. The softer handoff occurs when the mobile station moves between sectors of a single base station. In this type of handoff, a frequency and a frame offset are maintained, and the mobile station sets up a new communication channel with a destination sector while the present communication channel between the mobile station and the base station is maintained. Thus, a voice packet is transmitted on many channels. The initial communication channel is subsequently cut off when the mobile station has sufficiently moved into the destination sector such that maintenance of the initial communication channel is not required anymore.
The soft handoff occurs when the mobile station moves between adjacent base stations while a frequency and a frame offset are maintained. In this handoff, the mobile station sets up a communication channel with a destination base station while maintaining the intial communication channel with the first base station. A voice packet is thus transmitted on many channels. The initial communication channel is subsequently cut off when the mobile station has sufficiently moved into the destination cell such that maintenance of the initial communication channel is no longer required.
The hard handoff is used when the mobile station moves to an adjacent base station and the frame offset is changed or the frequency is changed. Additionally, the hard handoff is used when the mobile station moves to another switching center. Unlike the soft handoff and the softer handoff, the mobile station cannot maintain the initial communication channel. Rather, a newly set up communication channel is used to transmit the voice packet.
The soft and softer handoff in which both base stations use the same frequency proceed smoothly and without call cutting because there is no ping-pong effect that is typical with a hard handoff. The soft or softer handoff, however, cannot be used when a handoff is between different systems or between 2G˜3G systems having different frequencies, because the ping-pong effect caused by the handoff between the different frequencies leads to a very high call cut-off ratio.
In order to process the handoff between different CDMA systems or between 2G˜3G systems, each of the base stations of the systems must have a pilot beacon for providing information on a handoff starting time point to the base stations and the mobile station, respectively. That is, when a mobile station moves from a cell ‘A’ to a cell ‘B’, the cell ‘B’ pilot beacon transmits a pilot signal having a frequency used in cell ‘A’ at a strength identical to a signal strength at a boundary of cell ‘B’ for handoff switching time point from cell ‘A’ to cell ‘B’. Similarly, the cell ‘A’ pilot beacon transmits a pilot signal having a frequency used in cell ‘B’ at a strength identical to a signal strength at a boundary of cell ‘A’ for handoff switching time point from cell ‘B’ to cell ‘A’. However, in an overlap area between cell ‘A’ and cell ‘B’, the call is cut off due to the ping-pong of the handoff.
FIG. 1 illustrates a region between cells in a related art CDMA mobile communication system. Referring to FIG. 1, when the mobile station moves from cell ‘A’ to cell ‘B’, the cell ‘B’ pilot beacon transmits a pilot signal having a frequency used in cell ‘A’ at a strength equal to a signal strength at a boundary of cell ‘B’. The pilot beacon is transmitted to give the mobile station information on a handoff starting time point, for handing-off from cell ‘A’ to cell ‘B’. When the mobile station moves from a cell ‘B’ to a cell ‘A’, the cell ‘A’ pilot beacon transmits a pilot signal having a frequency used in cell ‘B’ at a strength equal to a signal strength at a boundary of cell ‘A’ for handing-off from cell ‘B’ to cell ‘A’.
FIG. 2 illustrates a related art method for performing a handoff between CDMA mobile communication systems. Referring to FIG. 2, a mobile station initially has a communication channel established through a base station of cell ‘A’ (step 200). When the mobile station moves from cell ‘A’ to cell ‘B’, if a strength of a pilot signal transmitted from a pilot beacon of cell ‘B’ and having a frequency identical to a frequency of the cell ‘A’ pilot signal meets a handoff threshold condition, the mobile station transmits a Pilot Strength Measurement Message (PSMM) to the base station of cell ‘A’ (step 201). Then, the cell ‘A’ base station analyzes the PSMM, and conducts an inter-frequency hard handoff to the cell ‘B’ base station (step 202). When the cell ‘A’ base station completes the inter-frequency hard handoff to the cell ‘B’ base station, the initial communication channel established between the mobile station and the cell ‘A’ base station is switched from the cell ‘A’ base station to the cell ‘B’ base station (step 203).
However, in a boundary area between cell ‘A’ and cell ‘B’, if the strength of the pilot signal used for processing a handoff varies continuously, the communication channel is switched from cell ‘B’ back to cell ‘A’. To do this, the mobile station transmits the PSMM to the cell ‘B’ base station when the strength of the pilot signal having a frequency identical to that of cell ‘B’ transmitted from the cell ‘A’ pilot beacon area meets the handoff threshold value condition (Step 204). Then, the inter-frequency hard handoff is made from cell ‘B’ to cell ‘A’ (step 205). Consequently, the communication channel established between the mobile station and the cell ‘B’ base station is set up between the mobile station and the base station of the cell ‘A’ (step 203).
The related art method for performing a hard handoff between different CDMA mobile communication systems thus has various problems. For example, it causes the ping-pong of handoff in which calls are repeatedly connected and cut-off. This is due to the ceaseless variation of the signal strengths of frequencies of the cells in the boundary area in handoff process, which causes signal cut off in the boundary area of systems. The frequent occurrence of the ping-pong effect during the handoff puts a heavy load on the PSMM and thus deteriorates communication quality. Additionally, it requires an additional device to process handoff of a data call owing to difficulty to find a control path or a bearer path between the systems.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.